Through-holes of components, for example laser-drilled holes, in particular cooling-air holes of gas turbine blades, often have complex geometries that differ from a cylindrical form. The diameter of the hole that is effective for flow, the location of the hole in the wall of the component, the position and the location and the offset of the diffusors (outflow region widened in cross section) of these holes vary on account of tolerances of the casting, laser or erosion process for example, or on account of the respective production conditions.
The effectiveness of the cooling-air bores on the airfoil profile of the turbine blade results from the complex interrelationship between these stated variables. Up to the present time, they cannot be determined or measured in an automated manner or without great technical expenditure.
With the conventional methods, the continuity of bores is checked by the detection of the heated component surface, i.e. if the hole is blocked, no heating of the material at the bore hole will occur. The disadvantage of this method is that a small opening (partial closure of the bore) also allows air to pass through and heat up the material. In a thermographic image it is scarcely possible to distinguish between partially closed bores and open bores.
Both DE 35 33 186 A1 and DE 197 20 461 A1 show thermographic methods in which a heated gas is forced through the cooling-air bores. The supply of warm air entails considerable expenditure on apparatus. The conventional thermographic method records the temperature distribution on the component surface which is heated by the warm air. However, conclusions concerning the form of the bore cannot be drawn from the information which can be obtained.